An display for displaying an image is one of essential information display mediums for modern life. The display for displaying an image is found as usage in several modes, such as starting from a so-called TV monitor, a liquid crystal display that has been rapidly developed recently, and an organic EL display that is expected the further development. Specifically, the liquid crystal display and the organic EL display are characterized by being able to drive in lower voltage, and are an important research and development issue in view of energy saving.
Above all, the organic EL display attracts most attention as a next generation flat panel display.
The emission mechanism of the light emitting element using an organic EL material used in the organic EL display is the mechanism of using a photon, in which a molecular exciton is formed with recombination of an electron injected from a cathode and a hole injected from an anode in the luminescent center of the electroluminescent film by being provided with a thin film (hereinafter referred to as an electroluminescent film) including a luminescent body between the electrodes and applying current to them, and then the photon is ejected when the molecular exciton returns to a ground.
Usually, the electroluminescent film is formed with a thin film having a film thickness of less than 1 μm. Further, the light emitting element using the organic EL material is a self-luminous type element in which electroluminescent film itself emits light, backlight used for the conventional liquid crystal display is not required. Therefore, it is a big advantage that the light emitting element can be manufactured extremely thinner and lighter.
As described above, the emission mechanism of the light emitting element using the organic EL material is the mechanism of using a photon, in which a molecular exciton is formed with recombination of an electron injected from a cathode and a hole injected from an anode in the luminescent center of the electroluminescent film by being provided with the electroluminescent between the electrodes and applying the current to them, and the photon is ejected when the molecular exciton returns to a ground. Therefore, it is one of requirements to efficiently inject the hole and the electron into the electroluminescent film for manufacturing an efficient light emitting element.
As a typical operation condition of the light emitting element using an organic EL material, current of approximately 100 mA/cm2 is applied to the electroluminescent film with originally higher electrical resistance. In order to realize the current injection having such a higher density, a hole injection barrier from the anode and an electron injection barrier from the cathode are to be reduced as far as possible. Namely, a metal with a smaller work function is to be used as the cathode; on the other hand, a metal with larger work function is to be used as the anode. As for the cathode, the work function can be practically controlled voluntarily by selecting various metal or alloy. On the contrary, the transparency is required for the anode in the light emitting element using the general organic EL material, the material for the light emitting element is limited to the transparent conductive oxide as it now stands. Considering the stability, the transparency and the resistivity, several oxide conductive films typified by indium-tin oxide (hereinafter referred to as ITO) must be selected for the anode.
The ITO electrode is the one doped (added) with tin into the indium oxide, and the tin gets into the substitution site of indium. The tin and the some oxygen defect become the donor, and conductivity is developed by partly filling the conductive band. The ITO is formed over a glass by sputtering, ion beam sputtering, vapor growth method, or the like. The lower resistance electrode with higher transparency can be manufactured by choosing the adequate amount of tin doping.
However, because the surface of the ITO film is not always flattened, it is pointed out that the contact with an electroluminescent film used for the light emitting element using the organic EL material is deteriorated or a pin hole is generated in the electroluminescent film. And it is said that is a reason of the deterioration of the light emitting element using the organic EL material. Further, the work function of the ITO film can be varied to some extent according to record in the film formation or the surface treatment, however the technique thereof is limited. Consequently, the reduction of the hole injection barrier is prevented.
As a method for reducing the hole injection barrier from ITO film as the anode, the technique to deposit a buffer layer over the ITO film is known. To make the adequate ionization potential of the buffer layer, the hole injection barrier can be reduced. The buffer layer is referred to as a hole injection layer. A substance that functions as the hole injection layer can be roughly categorized by metal oxide, low molecular organic compound, and high molecular compound. A conjugated polymer such as polyaniline (refer to non-patent literature 1) and polythiophene derivative (refer to non-patent literature 2) are known as the high molecular compound material. The conjugated polymer such as the polyaniline and the polythiophene rarely has conductivity by itself. And by mixing with strong acid such as camphor sulfonic acid and poly (styrenesulfonic acid) as acceptor, namely by doping, higher conductivity is developed. Accordingly, the conductive conjugated polymer which has been doped functions as the hole injection material.
(Non-Patent Reference. 1)
Y. Yang et al., Applied physics letters, Vol. 64, 1245 page, 1994
(Non-Patent Reference. 2)
S. A. Carter et al., Applied physics letters, Vol. 70, 2067 page, 1987
By using the conductive conjugated polymer which has been doped as the hole injection layer, the hole injection barrier is reduced and the hole can be efficiently injected. As a result, the efficiency and the lifetime of the light emitting element using the organic EL material can be improved, and the drive voltage can be reduced. The conductivity conjugated polymer is characterized by being able to be formed over the ITO surface in wet application method or ink-jetting.
A method of obtaining hydrophilic property of the substrate by using UV ozone rinsing or oxygen plasma rinsing is widely conducted so that the conductive conjugated polymer is uniformity formed in wet process against the substrate surface having the ITO.